P63 is a member of the p53 family. When the upstream promoter is used for p63 expression, three major TAp63 isoforms, TAp63a, TAp63, and TAp63?, are produced. p63 is also expressed from an alternate promoter located in intron 3, producing three major ?Np63 isoforms, ?Np63a, ?Np63, and ?Np63?3. As a transcription factor, TAp63 and ?Np63 are capable of regulating both common and distinct groups of target genes. Due to its sequence similarity with p53 and its transcriptional activity, p63 has many p53-like functions, such as an ability to induce cell cycle arrest, apoptosis, differentiation, and senescence. Thus, p63 is a tumor suppressor. Consistent with this, p63 status has been linked to increased survival rates and loss of p63 expression has been linked to increased potentials in metastasis. Likewise, mice heterozygous for p63 developed an increased tumor burden and metastasis rate, which was compounded in mice harboring heterozygous alleles of p53 and/or p73. However, p63 is not a classic tumor suppressor since p63 is expressed primarily in epithelial cells and appears to be necessary for the differentiation, maintenance, and regeneration of epithelial stem cells. In addition, mutant mice have developmental abnormalities. In an effort to characterize p63 functional domains and transcriptional activity, we identified and characterized two p63 targets: RNPC1, a RNA-binding protein, and Dec1, a basic helix-loop-helix transcription factor. Interestingly, we also found evidence that both RNPC1 and Dec1 are capable of regulating p63 expression. Considering that some of the stress signals, such as DNA damage and hypoxia that stabilize and activate p53, are found to have an effect on p63 expression, it is still not clear how p63 expression is regulated. Thus, we hypothesize that RNPC1 and Dec1 are both an effector and a modulator of the p63 pathway. To test this, we will determine (1) whether various p63 isoforms are differentially regulated by RNPC1 and Dec1 and (2) the mechanism by which p63 expression is regulated by RNPC1 and Dec1. PUBLIC HEALTH RELEVANCE: P63 is a member of the p53 tumor suppressor family and known to play a role in tumor suppression. The proposed study is to determine how p63 expression is regulated by RNPC1 and Dec1, both of which are also regulated by p63. Listed below are some of the rationales why the proposed study is significant. First, given the fact that p63 is found to play a critical role in tumor suppression but very little is known about its expression, the proposed study will provide an insight into the mechanism by which various stress signals impact on p63 expression. Second, p63 is expressed as two major isoforms, TAp63 and ?Np63, which are differentially expressed in various cell types and tissues. In addition, these p63 isoforms have distinct transcriptional and biological activities. Thus, how these p63 isoforms are differentially regulated by RNPC1 and Dec1 may provide an insight into the mechanism by which the expression patterns and biological activities of p63 are regulated. Third, ?Np63 is found to be amplified and/or over-expressed in some tumors, such as head and neck squamous cell carcinoma, and correlated with tumor progression and poor prognosis. Thus, the regulation of ?Np63 expression by RNPC1 and Dec1 can be explored as a potential therapeutic strategy to manage tumors with amplified and/or over- expressed ?Np63.